The invention relates to a circuit arrangement for operating one or more low-pressure discharge lamps. In particular, this is a circuit which detects breakage of a filament of a lamp and puts the circuit arrangement into a safe mode.
The service life of a low-pressure discharge lamp fitted with filaments is determined chiefly by the service life of the filaments. If the filaments are consumed, there is firstly an increase in lamp voltage in association with an undesired temperature increase in the filament region of the lamp. The lamp mostly also exhibits a rectifying effect in this stage. Finally, the filament breaks, and this can lead to destruction of the lamp operating device and to dangerous overheating of the lamp ends. Some disconnection devices are known for safe operation of the lamp and protection of the operating device:
The lamp voltage is frequently used in order to obtain a criterion for disconnecting the operating device, (for example, EP 0 809 923). However, even in normal operation, the lamp voltage is subject to strong fluctuations, and so it is impossible in many cases to specify a unique threshold at which disconnection is to be undertaken. The operating device mostly includes what is termed a coupling capacitor, which absorbs the direct component of the output voltage of the AC voltage generator included in the operating device. The voltage across the coupling capacitor is used in U.S. Pat. No. 5,493,181 to detect the abovementioned rectifying effect of the lamp. It is necessary in this case to arrive at a quantitative statement on the value of this voltage and compare it with a threshold. It is also valid here that the value of the voltage to be measured is subject in normal operation to strong fluctuations, and so it is frequently impossible to specify a unique threshold. Reliable disconnection is therefore impossible in many cases, or very complicated technically.
It has also emerged that monitoring the filaments with regard to breakage suffices in order to be able to ensure reliable operation of the system of lamp and operating device. In known solutions, it is detected whether a DC test current can flow through the filaments to be tested (DE 3805510). The disadvantage of this method is that the test current flows in addition to the current required for normal operation, and thus constitutes an additional load for the filaments.
During dimmed operation, in particular, the filaments are subjected to an additional heating current, over and above the current for the gas discharge. There are solutions for detection of filament breakage which monitor the presence of the additional heating current (EP 0 422 594). However, the additional heating current is frequently very small compared with the current for the gas discharge for which reason detection is complicated and unreliable.
It is the object of the present invention to provide a disconnection device for an operating device which accomplishes reliable disconnection of the operating device in the event of breakage of a filament, doing so with a low outlay.
This object is achieved in the case of a device having the features of the preamble of claim 1 by means of the features of the characterizing part of claim 1. Particularly advantageous refinements are to be found in the dependent claims.
Many operating devices for gas discharge lamps contain an AC voltage generator which outputs at its output a voltage which has a direct component. A half-bridge circuit which includes two controlled switches connected in series can be used to implement the AC voltage generator. However, it is mostly lamps which ought not to conduct direct current which are operated with the aid of these operating devices. Consequently, in addition to other components, the lamp is connected as a rule to the AC voltage generator via what is termed a coupling capacitor. It is important for the disconnection of the operating device according to the invention that the current for the gas discharge of the lamp is fed at only one end of a filament. The coupling capacitor absorbs the DC voltage component of the AC voltage source. This DC voltage component can be filtered out via an averaging unit for the purpose of disconnecting the operating device in accordance with the invention. A simple design of the averaging unit is a first-order low pass filter which, in the simplest case, comprises only a resistor and a capacitor. The DC voltage component of the coupling capacitor is now fed to a circuit part (denoted by SD below) which is responsible for the disconnection and has a threshold characteristic at its input. It is important that this feeding takes place via a filament. In the event of filament breakage, the DC voltage component of the coupling capacitor is absent at the input of the circuit part SD. The threshold characteristic at the input of the circuit part SD need only be capable of detecting the DC voltage component of the coupling capacitor. This can be implemented very reliably without great outlay. However, it is to be noted that apart from the direct component of the coupling capacitor no further DC voltage component is fed to the input of the circuit part SD.
The threshold characteristic can be implemented by a transistor. If a voltage is present at its input, it prevents charging of a capacitor (denoted below by C7) which is connected, for example, via its output terminals. If, in the event of filament breakage, there is no input voltage, the capacitor C7 is charged up and triggers disconnection of the operating device. The capacitor C7 is discharged when the operating device is taken into use. It thereby prevents an undesired disconnection during the starting operation of the lamp. The value of the capacitance of the capacitor C7 must be selected so large that disconnection can be triggered only after the DC voltage component at the coupling capacitor has stabilized in event of an intact lamp. If the DC voltage component is established, this is also an indication that the lamp has started properly. The DC voltage component at the coupling capacitor can therefore also be used to detect xe2x80x9clamp burningxe2x80x9d.
The disconnection of the operating device can be performed by a further controlled switch. When the further switch is triggered, the above-named AC voltage generator is turned off. This can be performed in various ways. Mostly, an auxiliary voltage is required to generate trigger signals in the AC voltage generator. With the aid of said further switch, the auxiliary voltage of the AC voltage generator can be suppressed, thereby achieving disconnection of the operating device. Some AC voltage generators have a separate input at which a signal must be present in order to disconnect the output signal of the AC voltage generator for safety purposes (safety disconnection signal). This safety disconnection signal can also be suppressed with the aid of said further switch for the purpose of disconnection.
The above-described circuit arrangement according to the invention for detecting filament breakage is suitable first and foremost for only one filament or for filaments of a plurality of lamps which are connected in parallel and are all at the same potential. If, additionally, filaments are to be monitored which are at a different potential, this can be done in a different way including using methods which are already known from the prior art. In order to be able to ensure absolutely safe operation of a lamp, it is necessary to monitor all the filaments, since it cannot be foreseen which filament will break first. Since the filaments belonging to a lamp are at very different potentials, particularly in the case of starting, it is not possible, as a rule, to apply cost-effective implementations of the filament monitoring to all the filaments simultaneously. In this context, filament monitoring according to the invention permits combination with other monitoring methods. Thus, for example, filaments which are not monitored according to the invention by detecting the DC voltage component at the coupling capacitor can be monitored in a different way. If the AC voltage generator requires an auxiliary voltage, this can be conducted via the filaments which have not as yet been monitored. In the event of breakage of these filaments, feeding of the auxiliary voltage is interrupted and the AC voltage generator is disconnected.
A further possibility for monitoring filaments not monitored so far consists in detecting the AC voltage components at a lamp terminal. As in the case of the detection of the DC voltage component, the current for the gas discharge of the lamp is fed only at one end of the filament to be detected. The AC voltage present at the other end of this filament is coupled out via a capacitor. If the filament breaks, the amplitude of the coupled-out AC voltage is substantially reduced. This can be utilized according to the invention in order to permit the capacitor C7 to be charged to a value which, as described above, leads to disconnection of the operating device. This is preferably performed by disturbing the discharge of the capacitor C7 by means of a further controlled switch.
In addition, the following requirement is frequently placed on the disconnection of an operating device: if the lamp is changed after completed disconnection, the aim thereby is to reverse the disconnection and permit operation of the new lamp. This is accomplished according to the invention by virtue of the fact that the charging current of the capacitor C7 is conducted via one or more filaments. If the lamp is removed, the capacitor C7 is discharged. If the voltage across the capacitor C7 undershoots a prescribed value, the disconnection is reversed.
Implementing this inventive idea requires a distinction to be made between AC voltage generators which are externally excited and those which are self-excited. Externally excited AC voltage generators have for the purpose of triggering the circuit breaker an oscillator which requires an auxiliary voltage. In order to detect breakage of a filament in a way not performed by detecting DC voltage across the coupling capacitor, it is possible, as described above, for said auxiliary voltage to be conducted via the filament to be checked. The charging of the capacitor C7, whose voltage is used for the disconnection, can also be performed via the same filament. Firstly, in the event of breakage of this filament, the oscillator is turned off, and thus the operating device is disconnected; secondly, in the event of a change of lamp, charging of the capacitor C7 is interrupted and the disconnection is thereby reversed.
Self-excited operating devices do not have a separate oscillator. The trigger signal for the circuit breaker is obtained from the load circuit. Consequently, there is no possibility of disconnecting the oscillator by means of interrupting the auxiliary voltage in the event of filament breakage. According to the invention, in this case, in the event of breakage of the filament which is not being monitored by the DC voltage level across the coupling capacitor, disconnection can be performed by means of the above-explained detection of the AC voltage component. However, this filament should not then bear the charging current of the capacitor C7 on its own. The breakage of the filament would then certainly be detected, but the subsequent charging of the capacitor C7 would be interrupted, for which reason no disconnection would come about. Consequently, according to the invention, both lamp filaments are used in order to provide the charging current for the capacitor C7. Irrespective of which filament breaks, it is therefore ensured that a charging current which leads to disconnection is provided for C7. This AND operation of the filament currents is achieved by virtue of the fact that the lamp terminals which are not fed by the AC voltage generator are connected in each case to the capacitor C7 via a diode.
In this context, it is necessary to mention another aspect of the operating device with self-excited AC voltage generator. In particular, in the case of the AC voltage generator with a half bridge, importance attaches as to which state of charge the capacitors have on the occasion when the circuit breaker is first closed. The capacitors must be charged such that this first closing of a circuit breaker effects a flow of current which brings about the self-excitation of the AC voltage generator. The charge relationships of the capacitors can be displaced before starting the AC voltage generator by means of the two said diodes for AND operation. If appropriate, it is necessary to modify the starting circuit whose task is to close one of the two half-bridge switches once. This modification can be such that it is no longer the lower half-bridge switch, but the upper half-bridge switch which is first closed.